1. Field of the Invention
The present invention relates to an image display apparatus using an image display element such as a liquid crystal panel, in particular a reflective image display element.
2. Related Background Art
U.S. Pat. No. 5,327,270 discloses an image projecting apparatus using a reflective liquid crystal display element, which is structured so that light coming from a light source is led to a reflective liquid crystal display element via a polarization beam splitter, the light reflected by the reflective liquid crystal display element is again detected and projected by the polarization beam splitter, in which a ¼ phase plate is provided between the polarization beam splitter and the reflective liquid crystal display element to compensate the contrast (FIG. 6).
In order to brighten an image projected by the image projecting apparatus, it is necessary that a luminous flux dividing system which illuminates an illumination area at a uniform illuminance and a condensing optical system including a polarization splitting system which aligns the polarization direction of the illumination light, a condenser lens which condenses a plurality of luminous fluxes emerged from the polarization splitting system, and a field lens which makes light incident into the reflective liquid crystal display element into substantially telecentric light are provided. Further, in order to improve the utilization efficiency of an illumination system having the above systems, it is necessary to make small Fno (=f/L) which is determined by a combined focal length f of the condensing optical system and an effective range width L of the condenser lens, whereby it is necessary to further make large the maximum incident angle of the luminous flux condensed at an optional one point on the reflective liquid crystal display element.
However, since the ¼ phase plate is an anisotropic structure having a predetermined thickness, the phase difference which is produced by the incident angle differs. Where light having a polarization axis which is inclined by 45° with respect to the axes of an ordinary ray and an extraordinary ray propagates in the direction of the ordinary ray in an anisotropic substance in which the refractive index of the ordinary ray is no and the refractive index of the extraordinary ray is ne, the phase difference φ of the light is expressed as follows;
                    ϕ        =                                            2              ⁢                                                          ⁢              π                        λ                    ⁢                      (                                          n                o                            -                              n                e                                      )                    ⁢                                          ⁢          l                                    (        1        )            where λ is a wavelength and 1 is a thickness of a substance in which light propagates.
Based on the expression (1), where a difference of optical paths, which is generated in the ordinary ray direction and extraordinary ray direction when light having a wavelength λ passes through the substance is:
            (                        n          0                -                  n          e                    )        ⁢    l    =      λ    4  the phase difference becomes:
  ϕ  =            π      2        .  
Therefore, the light incident as linear polarized light into the substance is converted to circular polarization and the substance is made into a ¼ phase plate.
However, if light is made incident obliquely into the anisotropic substance, it is shown as an ellipse body having a refractive index no in the X-axis direction and Y-axis direction and a refractive index ne in the z-axis direction as shown in FIG. 5, wherein if the propagation direction (vector a) of a wavefront is inclined by α on a yz plane from the extraordinary ray direction, the refractive index no′ of the ordinary ray and refractive index ne′ of the extraordinary ray with respect to electric fields, which are perpendicular to each other, becomes as follows;
      n    e    ′    =      1                                                      cos              2                        ⁢            α                                n            o            2                          +                                            sin              2                        ⁢            α                                n            e            2                              no′=no  (2)
and, the phase difference φ′ of light is expressed by:
                              ϕ          ′                =                                            2              ⁢                                                          ⁢              π                        λ                    ⁢                      (                                          n                o                            -                              1                /                                                                                                                              cos                          2                                                ⁢                        α                                                                    n                        o                        2                                                              +                                                                                            sin                          2                                                ⁢                        α                                                                    n                        e                        2                                                                                                                  )                    ⁢                                          ⁢                                    l                              sin                ⁢                                                                  ⁢                α                                      .                                              (        3        )            
Therefore, the phase difference φ depends on the incident angle α, wherein the incident angle with respect to the ¼ phase plate is deviated from the reference incident angle (α=90°), and the phase difference φ′ expressed by the expression (3) becomes greatly different from a value obtained by the expression (1). Then, it is not possible to obtain sufficient contrast even by using the ¼ phase plate.